Q: All my exterior lights work (brake, signal, hazard, reverse, etc.) EXCEPT running tail lights

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My issue is that all my lights work - blinkers, hazards, reverse, brake, fog, etc. except the running tail lights. The biggest issue is that I've looked everywhere and no one seems what it is. I've check for ground (DONE), I've check for fuses (DONE), I've check for bad bulbs (DONE), I've check pretty much everything. My biggest issue is that the wire that goes to the left and right Taillamp is the same that goes into the brake light... therefore when I push in the brake light I get 12V, otherwise (with the lights on) I get 3.3V. Please help... people say I have a back electronic control module (cost $450) but other say it could be a relay.

My car has 141000 miles.
My car has a manual transmission.

If you are certain all fuses are good/tight, grounds are good and all bulbs are correct in voltage for the vehicle, I have a suggestion from personal experience with a MK 5 Jetta sitting right outside. Make sure the front marker lights are working. The issue we were experiencing with this Jetta was similar. When switching the lights on, the left rear marker would light for a brief moment, then go out and set a bulb fault on the instrument cluster.

After spending a bit of time on that, we found the left FRONT marker light in the headlight housing was out. The bulb was found to be good upon removal and inspection, but the terminals had a bit of corrosion, giving a poor ground. After cleaning the contacts with a bit of emery cloth and reinstalling the socket, both front and rear bulbs are working properly.

If you need some help with this, consider YourMechanic, as a mobile expert can come to you and diagnose your lights firsthand. Best of luck.

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